1. Field of the Invention
The present invention relates to wireless communications systems, and signal processing apparatus employed in wireless communications systems. More particularly, the present invention relates to communications systems that transmit simultaneously on multiple carriers.
2. Background of the Prior Art and Related Information
Wireless communications systems employing transmission between base stations and remote users are a key component of the modern communications infrastructure. These wireless systems are being placed under increasing performance demands which are taxing the capability of available equipment, especially wireless base station equipment. These increasing performance demands are due to both the increasing numbers of users within a given wireless region, as well as the bandwidth requirements allocated to wireless system service providers. The increasing number of wireless users is of course readily apparent and this trend is unlikely to slow due to the convenience of wireless services. The second consideration is largely due to the increased types of functionality provided by wireless systems, such as wireless Internet access and other forms of wireless data transfer over such systems. These considerations have resulted in a need for more carriers operating from each transmitting location of a wireless service network.
When transmitting multiple carriers from a single location, it is advantageous to combine carriers as early in the signal generation process as possible. By doing so, the transmitting location can reduce the number of antennas, low loss cables and power amplifiers. The ultimate goal is to combine individual channels with digital signal processing before RF signal generation. In some multiple carrier systems like OFDM (orthogonal frequency division multiple access), it is best to combine carriers with digital signal processing in order to maintain the required orthogonal carrier characteristics.
An example of a prior art multiple carrier signal generator is shown in FIG. 1. Information symbol streams are input from the individual carrier data handling systems. These symbols are generally complex valued data points that represent one or more information data bits to be transmitted. The symbol streams are then upsampled to a higher sample rate by inserting equally time spaced zero valued samples between the input symbols. The upsampled symbol streams are then passed through baseband filters to create the individual carrier baseband signal waveforms. After the individual carrier baseband signals are created, each carrier is offset in frequency and combined producing a multiple carrier communication signal. This signal is then digital-to-analog converted and up-converted in frequency to the desired operating bandwidth.
A problem exists, however, with the above multiple carrier generation process. Individual carriers in the above process obey power statistics that are a function of the symbol generation process used and the baseband filter impulse response function characteristics. These individual carrier power statistics are generally described by the carrier power complementary cumulative probability density function (CCDF), and from that function, the signal power peak-to-average ratio. Signals with high peak-to-average ratios create problems in digital-to-analog conversion, RF signal generation, and RF power amplification. Even if the peak-to-average power ratios of individual carriers is low, or can be maintained below a maximum level using signal-processing algorithms, combining these multiple carriers may produce high peak-to-average ratios. These high multi-carrier peak-to-average ratios once again cause problems in digital-to-analog conversion, RF signal generation, and RF power amplification.
Signals with high peak-to-average ratios cause the following problems in communications systems. First, the number of significant digits used to calculate the signal must be large enough to maintain adequate signal resolution when the signal is both very large and very small. Second, at the output of the digital signal processor are digital-to-analog converters. To accommodate a digital signal with a high peak-to-average ratio, high bit count digital-to-analog converters must be used so that both large and small values can be generated. If this is not done the output signal will have a poor output signal-to-noise ratio. Finally, signals with high peak-to-average ratios require very linear analog, IF, RF and RF power amplifier circuits. Without these very linear circuits, distortion products are generated at frequencies outside the government allocated bandwidth of the wireless system license. Such highly linear circuits are expensive, however, and add considerable cost to the system.
Accordingly, a problem presently exists in multiple carrier communication systems due to large peak-to-average power ratios occurring after carrier combination.